The IS/IP claim is that it/they provide a superior way to manage confrontations, generate physical power and more. It is then natural to ask why these abilities aren't available to humans "by default" and require a fundamental retraining of the body.
If anything, it may be rather a measure of the unintended abuse humans put upon their bodies and the warped line of evolution through which we arrived at our peculiar form. Our bodies are evolutionary overkill, because they had to be to survive everything a silly monkey could do to it. That overkill can be exploited, but it is not very easy and requires a long and focused period of learning.
That's no different than elite level performance in any physical or mental endeavor. People are not just born sprinters or swimmers or cyclists or gymnasts or writers or astrophysicists or violinists. Sure, you can try to do any of those things without practice, you just will suck at it relative to what is possible. You have to actually develop your innate biological potential, it doesn't just happen on its own. We can already move, balance, absorb and generate power, feel, react, blah blah blah, without training, just rarely good enough to deal with the unpredictable nature of monkey-on-monkey combat.
Also modern living style is mostly sedentary, so we spend a lot of time conditioning in bad movement and thinking habits and postural perversions. We also learn bad movement habits from our parents, who also most likely learned bad movement habits. Half the practice is just removing all the bad crap you've accumulated before starting training, which was allowed to develop because it was never physically stressed enough to fail, which, as far as one-trial human learning goes, gets interpreted as success.